The present invention relates to electrodes for electrolytic capacitors and, to a method for producing such electrodes, particularly anodes but also cathodes, having increased effective surface area. The invention moreover relates to anodized electrodes which comprise non-cylindrical pores having a branched morphology.
Electrolytic capacitors are elementary electrical devices, intended to accumulate a static electric charge on their plates. One plate is metallic and the other plate is an electrolyte. Intervening between the two plates is a dielectric consisting of a surface oxide coating on the metal plate. Conventionally, the metal plate on which the dielectric coating is formed is referred to as the anode. The term “anode” is used herein to refer both to the metal plate itself and to the combination of the metal plate with the dielectric coating. It will be clear from the context which meaning of “anode” is intended.
To increase substantially the surface area of a metal electrode, and consequently the value of its specific capacitance, the etching process is carried out electrolytically with a chloride solution which dissolves metal and increases the surface area of the foil, forming a dense network like innumerable microscopic channels (pits). See for example U.S. Pat. No. 4,537,665 (Nguyen, et al.) which describes the manufacture of low-voltage aluminum foil electrolytic capacitor electrodes including etching the foil, cleaning and anodizing it, the cleaned foil being thermally treated at about 595-650° C., and then anodizing the treated foil in an adipate electrolyte.
In other prior etching technologies, the etching solutions typically contain Cl− ions, see e.g. U.S. Pat. No. 6,168,706 (Hemphill, et al.), which employs hydrated AlCl3, HCl, H2SO4 and HClO4 or ClO4−. It is believed, however, that the presence of halogens in etching solutions is liable to cause serious ecological problems, complex and high cost disposal and in addition inhibit anodization.
In US 20020149902 A1, published Oct. 17, 2002, there is described an electrode foil for an aluminum electrolytic capacitor, comprising a plurality of main pits formed by etching on both surfaces of an aluminum foil to extend from the surfaces in a thickness direction of the foil, and sub pits branched away from the main pits and along the length of the main pits, as well as a multi-step etching procedure for creating and enlarging both main pits and sub pits. In practice, the main pits are vertical and have a uniform and apparently cylindrical cross section, while the sub-pits are formed perpendicularly to the longitudinal axis of the main pits and appear also to have a uniform and apparently cylindrical cross section. Use in this procedure of chlorine-containing etchants necessitated a dechlorination step.
To overcome such disadvantages and also to reduce capacitor size by using thinner Al foil recently, vacuum deposition has been proposed for increasing the surface area of foil electrodes. Thus, e.g., Drake, in U.S. Pat. No. 4,309,810, teaches vacuum deposition of a metal vapor at a low angle onto a foil substrate, and presents an example of the deposition of aluminum on aluminum to give a columnar structure. However, Drake's foil has been found to be too brittle for use in electrolytic capacitors, because the columns break when the foil is rolled into a cylindrical roll, one of the standard procedures in the manufacture of electrolytic capacitors.
Another method of electrode manufacture by an improved vacuum deposition method, providing high surface area values, has been described by Katsir et al., in U.S. Pat. No. 6,287,673. This method utilizes condensation of metal vapor on a thin conducting substrate in an atmosphere of low-pressure inert gas/oxygen mixture, so as to produce a porous coating structure having a high surface area. Aluminum is generally a suitable material to be selected from the family of valve metals, due to its high electrical conductivity, low cost of high-purity aluminum foil and bulk metal, good dielectric properties of alumina, and other technological advantages. The required oxide film is produced by electrolytially anodizing the obtained coating, by DC current in a nonsolving or weakly solving electrolyte, such as the ammonium salts of boric, citric, oxalic and adipic acids or mixtures thereof.
The process of anodizing foils is accompanied, unavoidably, by gradual filling of its pores by oxide due to increasing thickness of the alumina layer, and of course filled pores do not contribute positively to the surface area. It is clear that the width and depth of pores are critical parameters of the obtained coating. If the initial sizes of trench-like pores are not large enough, the process of filling the pores during anodization leads to rapidly decreasing capacitance of the desired electrodes. On the other hand, the capacitance also decreases if the pore size is too large. For anode foils, it is necessary that pore diameter is substantially larger than the thickness of the dielectric coating (e.g. Al2O3) made by the anodization voltage. By way of example, a simplified estimation for Al2O3 thickness for 60 V forming voltage, i.e. the final voltage, is as follows: the Al2O3 thickness will be 14 Å per volt×60 volt=840 Å. Thus, it is necessary that—in this illustration—pore diameter will be larger than 840 Å (×2), and pore sizes with a smaller diameter will not contribute to the capacitance because the pores will be filled up by Al2O3. As the thickness of the dielectric layer increases, the capacitance decreases, but with a consequent increase in the working voltage of the capacitor.
Vacuum deposition methods per se are limited in their ability to create large pores. Therefore, high capacitance can be obtained only for an extremely low-voltage range using such known methods.
It would thus be desirable to provide a method free from the usage of Cl for manufacturing electrolytic capacitor electrodes and to obtain (for example) anode foils for aluminum electrolytic capacitors with increased values of specific capacity, suitable for a broader in-use voltage range up to and more than 100 volts.
It is accordingly an object of the invention to provide electrolytic capacitor electrodes based on vacuum deposition techniques, but with increased values of specific capacity, and a method for making such electrodes.
Other objects of the invention will appear from the description which follows.
The entire contents of the above-mentioned U.S. Patents and published U.S. Patent Application are incorporated by reference herein.